EL display device and manufacturing method thereof

ABSTRACT

Reducing the manufacturing cost of an EL display device and an electronic device furnished with the EL display device is taken as an objective. A textured structure in which projecting portions are formed on the surface of a cathode is used. External stray light is diffusely (irregularly) reflected by the action of the projecting portions when reflected by the surface of the cathode, and therefore a defect in which the face of an observer or the surrounding scenery is reflected in the surface of the cathode can be prevented. This can be completed without using a conventionally necessary high price circular polarizing film, and therefore it is possible to reduce the cost of manufacturing the EL display device.

This application is a continuation of U.S. application Ser. No.12/758,566 filed on Apr. 12, 2010 now U.S. Pat. No. 8,198,806 which is acontinuation of U.S. application Ser. No. 12/208,528 filed on Sep. 11,2008 (now U.S. Pat. No. 7,710,028 issued May 4, 2010) which is acontinuation of U.S. application Ser. No. 10/943,089, filed on Sep. 16,2004 (now U.S. Pat. No. 7,427,834 issued Sep. 23, 2008) which is acontinuation of U.S. application Ser. No. 10/384,807 filed on Mar. 10,2003 (now U.S. Pat. No. 7,012,300 issued Mar. 14, 2003) which is acontinuation of U.S. application Ser. No. 10/186,398, filed on Jul. 1,2002 (now U.S. Pat. No. 6,555,969 issued Apr. 29, 2003) which is acontinuation of U.S. application Ser. No. 09/644,429, filed on Aug. 23,2000 (now U.S. Pat. No. 6,433,487 issued on Aug. 13, 2002).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electro-optical device, typically anEL (electroluminescence) display device, and an electronic device(electronic equipment) having the electro-optical device as a display.

2. Description of the Related Art

The development of electro-optical devices, typically EL(electroluminescence) display devices using organic material forelectroluminescence, has been proceeding at a rapid pace in recentyears. There are two types of EL display devices, passive matrix type ELdisplay devices and active matrix type EL display devices.

Regardless of whether a passive matrix type or an active matrix type,the EL display device has a capacitor structure with an EL layersandwiched by a cathode and an anode (an element having this type ofstructure is referred to as an EL element throughout thisspecification), and the EL display device operates under the principleof causing the EL layer to luminesce by the flow of electric current. Ametallic electrode is generally used for the cathode, which is anelectron supply source, and a transparent conducting film is generallyused for the anode, which is a hole supply source. This is done becauseif one of the pair of electrodes is not transparent, the light emittedfrom the luminescing layer cannot be extracted.

In this case, the light emitted by the EL layer is directly output tothe anode side, and light directed toward the cathode side is alsooutput to the anode side after being reflected by the cathode. In otherwords, it is necessary for an observer to view the display device fromthe anode side.

However, light having a wavelength corresponding to the material of theluminescing layer can be seen from a portion of the EL layer emittinglight, but in a portion of the EL layer not emitting light, the surfaceof the back surface side of the electrode (light emitting layer side)can be seen through the anode and the EL layer. This means that the backsurface of the electrode therefore functions as a mirror, and the faceof the observer is reflected.

In order to avoid this, a method of attaching a circular polarizationfilm to the EL display device so that the observer's face is notreflected is employed, but there is a problem in that the circularpolarization film is extremely high cost, therefore leading to increasedmanufacturing costs.

SUMMARY OF THE INVENTION

The present invention is made in view of the above problems, and anobject of the present invention is to prevent an EL display device frombecoming mirrored, and to provide a low cost EL display device in whichthe EL display device manufacturing cost has been reduced. In addition,an object of the present invention is to lower the cost of an electronicdevice having a display using the EL display device.

The present invention is characterized in that a projecting portion isformed on a reflecting surface of a cathode (a surface contacting aluminescing layer side), and light reflected by the reflecting surfaceof the cathode is scattered. Namely, the present invention ischaracterized in that the reflecting surface of the cathode is made notvisible to an observer by diffusely (irregularly) reflecting visiblelight (external light) incident from an anode side by using thereflecting surface of the cathode.

The textured portion formed on the reflecting surface of the cathode maybe formed by concave shape depressions, or by convex shape projections.Further, a wave shape surface in which the unevenness is repeated mayalso be used. The projecting portion may be formed by a technique suchas photolithography or holography (for example, a technique of formingan uneven reflecting structure recorded in Sharp Technology Reports, No.74, pp. 16-9, August 1999), and may also be formed by surfaceprocessing, such as plasma treatment or etching. Further, the projectingportion may also be naturally generated in the surface by using the filmdeposition conditions of the cathode (or a base electrode).

In other words, the formation of the projecting portion may be regulatedor unregulated, but it must be formed so as to average a diffusedreflection (irregular reflection) within the surface of a pixel. Astructure in which the projecting portion is formed as explained aboveis referred to as a textured structure throughout this specification.

Further, by forming projecting portions in other thin films contactingthe cathode, and then forming the cathode on top, the projecting portioncan be formed in the reflecting surface of the cathode. In particular,Japanese Patent Application Laid-open No. Hei 9-69642 and JapanesePatent Application Laid-open No. Hei 10-144927 can be cited for means offorming the projecting portion in an aluminum film. Namely, by formingthe aluminum film based on the above patent applications, and bylaminating the cathode on top of the aluminum film, it is possible toobtain a cathode having the projecting portion.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a diagram showing a cross sectional structure of a pixelportion of an EL display device;

FIG. 2 is a diagram showing an enlargement of an EL element;

FIGS. 3A and 3B are diagrams showing a top surface structure and acircuit structure of a pixel portion of an EL display device;

FIGS. 4A to 4E are diagrams showing a process of manufacturing an activematrix type EL display device;

FIGS. 5A to 5D are diagrams showing the process of manufacturing theactive matrix type EL display device;

FIGS. 6A to 6C are diagrams showing the process of manufacturing theactive matrix type EL display device;

FIG. 7 is a diagram showing an external view of an EL module;

FIG. 8 is a diagram showing a circuit block structure of an EL displaydevice;

FIG. 9 is an enlarged diagram of a pixel portion of an EL displaydevice;

FIG. 10 is a diagram showing the element structure of a sampling circuitof an EL display device;

FIGS. 11A and 11B are diagrams showing external views of an EL module;

FIGS. 12A to 12C are diagrams showing a process of manufacturing acontact structure;

FIG. 13 is a diagram showing the composition of a pixel portion of an ELdisplay device;

FIG. 14 is a diagram showing the composition of a pixel portion of an ELdisplay device;

FIG. 15 is a diagram showing an external view of a thin film formationapparatus;

FIG. 16 is a diagram showing an external view of a simple matrix type ELdisplay device;

FIGS. 17A to 17F are diagrams showing specific examples of electronicdevices;

FIGS. 18A to 18E are diagrams showing a process of manufacturing anactive matrix type EL display device; and

FIGS. 19A to 19D are diagrams showing a process of manufacturing anactive matrix type EL display device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment Mode 1

Embodiment mode 1 of the present invention will be explained using FIGS.1 to 3B. Shown in FIG. 1 is a cross sectional diagram of a pixel portionof an EL display device of the present invention, while FIG. 2 shows anenlargement of a portion of FIG. 1. FIG. 3A is a top view of the ELdisplay device, and FIG. 3B is a circuit diagram of the EL displaydevice. In practice, a plurality of pixels arranged in a matrix shape isformed as the pixel portion (image display portion). Note that commonsymbols are used in FIGS. 1 to 3B, and therefore each of the diagramsmay be suitably referred to. Further, two pixels are shown in the topview of FIG. 3A, but both have the same structure, and therefore onlyone is explained.

In FIG. 1, a reference numeral 11 designates a substrate; and 12, aninsulating film (hereinafter referred to as an under film) which becomesan undercoat. A glass substrate, a glass ceramic substrate, a quartzsubstrate, a silicon substrate, a ceramic substrate, a metal substrate,or a plastic substrate (including a plastic film as well) can be used asthe substrate 11.

As the under film 12, an insulating film containing silicon may be used.Note that in the present specification, the “insulating film containingsilicon” indicates an insulating film containing silicon, oxygen andnitrogen at a predetermined ratio, for example, a silicon oxide film, asilicon nitride film, or a silicon nitride oxide film (indicated bySiOxNy).

Here, two TFTs are formed in the pixel. A reference numeral 201designates a TFT (hereinafter referred to as a switching TFT)functioning as a switching element; and 202, a TFT (hereinafter referredto as a current controlling TFT) functioning as a current controllingelement for controlling the amount of current flowing to the EL element.Both are formed out of an n-channel TFT but a p-channel TFT may also beused.

The switching TFT 201 has: an active layer containing a source region13, a drain region 14, LDD regions (lightly doped regions) 15 a to 15 d,a high concentration impurity region 16, and channel forming regions 17a and 17 b; a gate insulating film 18; gate electrodes 19 a and 19 b; aprotecting film 20 made from a silicon nitride oxide film; a firstinterlayer insulating film 21; a source wiring 22; and a drain wiring23. Note that the drain region 14 is electrically connected to a gateelectrode 35 of the current control TFT 202 through the drain wiring 23.

Besides, the gate electrodes 19 a and 19 b are of a double gatestructure and also in addition to the double gate structure, a so-calledmulti-gate structure (structure including an active layer having two ormore channel formation regions connected in series with each other),such as a triple gate structure, may be adopted. The multi-gatestructure is extremely effective in reducing the off current value, andis an extremely effective structure as the switching element of a pixel.

The active layer is formed out of a semiconductor film containing acrystal structure. That is, a single crystal semiconductor film may beused or a polycrystalline semiconductor film or microcrystallinesemiconductor film may be used. The gate insulating film 18, theprotecting film 20 and the first interlayer insulating film 21 may beformed out of an insulating film containing silicon. Besides, anyconductive films can be used for the gate electrodes 19 a and 19 b,source wiring line 21, or drain wiring line 22.

Further, in the switching TFT 201, the LDD regions 15 a to 15 d areprovided not to overlap with the gate electrodes 17 a and 17 b, with thegate insulating film 18 put between the LDD regions and the gateelectrodes. Such structure is very effective in reducing the off currentvalue.

Note that it is more desirable to provide an offset region (region whichis made of a semiconductor layer having the same composition as thechannel formation region and to which a gate voltage is not applied)between the channel formation region and the LDD region in order toreduce the off current. In the case of multi-gate structure having twoor more gate electrodes, a high concentration impurity region providedbetween the channel formation regions is effective in reducing the offcurrent value.

As described above, by using the TFT of the multi-gate structure as theswitching TFT 201 of the pixel, it is possible to realize the switchelement having a sufficiently low off current value. Thus, even if acondenser as shown in FIG. 2 of Japanese Patent Application Laid-openNo. Hei 10-189252 is not provided, the gate voltage of the currentcontrolling TFT can be held for a sufficient time (an interval between aselected point and a next selected point).

That is, it becomes possible to remove a condenser which hasconventionally been a factor to narrow an effective light emitting area,and it becomes possible to widen the effective light emitting area. Thismeans that the picture quality of the EL display device can be madebright.

Next, the current controlling TFT 202 includes an active layer includinga source region 31, a drain region 32, an LDD region 33 and a channelformation region 34, a gate insulating film 18, a gate electrode 35, aprotecting film 20, the first interlayer insulating film 21, a sourcewiring line 36, and a drain wiring line 37. Although the gate electrode35 is of a single gate structure, a multi-gate structure may be adopted.

As shown in FIG. 1, the drain of the switching TFT is connected to thegate of the current controlling TFT. Specifically, the gate electrode 35of the current controlling TFT 202 is electrically connected to thedrain region 14 of the switching TFT 201 through the drain wiring line(may be called a connection wiring line) 23. The source wiring line 36is connected to a current supply line 211.

Although the current controlling TFT 202 is an element for controllingthe amount of current injected to an EL element 203, in view ofdeterioration of the EL element, it is not desirable to supply a largeamount of current. Thus, in order to prevent an excessive current fromflowing to the current controlling TFT 202, it is preferable to designthe channel length (L) to be rather long. Desirably, it is designed sothat the current becomes 0.5 to 2 μA (preferably 1 to 1.5 μA) per pixel.

In view of the above, as shown in FIG. 9, when the channel length of theswitching TFT is L1 (L1=L1 a+L1 b), the channel width is W1, the channellength of the current controlling TFT is L2, and the channel width isW2, it is preferable that W1 is made 0.1 to 5 μm (typically 0.5 to 2μm), and W2 is made 0.5 to 10 μm (typically 2 to 5 μm). Besides, it ispreferable that L1 is made 0.2 to 18 μm (typically 2 to 15 μm), and L2is made 1 to 50 μm (typically 10 to 30 μm). However, the presentinvention is not limited to the above numerical values.

Besides, it is appropriate that the length (width) of the LDD regionformed in the switching TFT 201 is made 0.5 to 3.5 μm, typically 2.0 to2.5 μm.

Besides, the EL display device shown in FIG. 1 is characterized also inthat in the current controlling TFT 202, the LDD region 33 is providedbetween the drain region 32 and the channel formation region 34, and theLDD region 33 includes a region overlapping with and a region notoverlapping with the gate electrode 35, with the gate insulating film 18put between them.

The current controlling TFT 202 supplies current for causing the ELelement 203 to emit light, and controls the supply amount to enablegradation display. Thus, it is necessary to take a countermeasureagainst deterioration due to the hot carrier injection so thatdeterioration does not occur even if current is supplied. When black isdisplayed, although the current controlling TFT 202 is turned off, atthat time, if an off current value is high, clear black display becomesimpossible, and the lowering of contrast or the like is caused. Thus, itis necessary to suppress the off current value as well.

With respect to the deterioration due to the hot carrier injection, itis known that the structure where the LDD region overlaps with the gateelectrode is very effective. However, if the whole of the LDD region ismade to overlap with the gate electrode, the off current value isincreased. Thus, the present applicant contrives a new structure thatthe LDD region not overlapping with the gate electrode is provided inseries, so that the problems of the hot carrier countermeasure and theoff current value countermeasure are solved at the same time.

At this time, it is appropriate that the length of the LDD regionoverlapping with the gate electrode is made 0.1 to 3 μm (preferably 0.3to 1.5 μm). If the length is too long, parasitic capacity becomes large,and if too short, the effect of preventing the hot carrier becomes weak.Besides, it is appropriate that the length of the LDD region notoverlapping with the gate electrode is made 1.0 to 3.5 μm (preferably1.5 to 2.0 μm). If the length is too long, it becomes impossible to makea sufficient current flow, and if too short, the effect of lowering theoff current value becomes weak.

In the above structure, parasitic capacity is formed in the region wherethe gate electrode and the LDD region overlap with each other. Thus, itis preferable not to provide such region between the source region 31and the channel formation region 34. In the current controlling TFT,since the direction of flow of carriers (here, electrons) is always thesame, it is sufficient if the LDD region is provided at only the side ofthe drain region.

Further, looked at from the viewpoint of controlling the amount ofelectrical current flow, it is also effective to make the film thicknessof the active layer (in particular the channel forming region) of thecurrent control TFT 202 thinner (preferably from 20 to 50 nm, evenbetter between 30 and 35 nm). Thus reducing the current flow value alsobrings about a desirable effect for the important switching TFT 201.

Next, reference numeral 41 denotes a first passivation film, and itsfilm thickness may be from 200 to 500 nm (preferably between 300 and 400nm). An insulating film containing silicon (a silicon nitride oxide filmor a silicon nitride film is particularly preferable) can be used as thefirst passivation film 41 material, which also possesses a role ofprotecting the formed TFTs. Mobile ions such as alkaline metals areoften contained in an EL layer formed last on the TFT, and the firstpassivation film 41 works as a protecting film so that the mobile ionsdo not enter the TFT side.

Furthermore, by giving the first passivation film 41 a heat radiatingeffect, it is effective in the prevention of heat degradation of the ELlayer and the TFTs. The following can be given as materials possessingthe heat radiating effect: an insulating film containing at least oneelement selected from the group consisting of B (boron), C (carbon), andN (nitrogen), and at least one element selected from the groupconsisting of Al (aluminum), Si (silicon), and P (phosphorous).

For example, it is possible to use a nitride of aluminum typified byaluminum nitride (AlxNy), carbide of silicon typified by silicon carbide(SixCy), nitride of silicon typified by silicon nitride (SixNy), nitrideof boron typified by boron nitride (BxNy), or phosphide of borontypified by boron phosphide (BxPy). An oxide of aluminum typified byaluminum oxide (AlxOy) has a thermal conductivity of 20 Wm⁻¹K, so thatit can be said as one of preferable materials. These materials have notonly the foregoing effects but also an effect to prevent penetration ofmoisture. Note that in the foregoing materials, x and y are respectivelyarbitrary integers.

Note that it is also possible to combine the above compound with anotherelement. For example, it is also possible to use aluminum nitride oxideindicated by AlNxOy by adding nitrogen to the aluminum oxide. Thismaterial also has the effect to prevent penetration of moisture oralkali metal in addition to the heat radiating effect. Note that in theabove aluminum nitride oxide, x and y are respectively arbitraryintegers.

Besides, it is possible to use materials disclosed in Japanese PatentApplication Laid-open No. Sho 62-90260. That is, it is also possible touse an insulating film containing Si, Al, N, O, or M (M is at least onekind of rare-earth element, preferably at least one element selectedfrom Ce (cerium), Yb (ytterbium), Sm (samarium), Er (erbium), Y(yttrium), La (lantern), Gd (gadolinium), Dy (dysprosium), and Nd(neodymium)). These materials also have the effect to preventpenetration of moisture or alkali metal in addition to the heatradiating effect.

Besides, it is also possible to use a carbon film containing at least adiamond thin film or an amorphous carbon film (especially a film havingcharacteristics close to diamond, called diamond-like carbon or thelike). These have very high thermal conductivity and are very effectiveas a heat radiating layer.

Note that since the primary object of the first passivation film 41 isto protect the TFT against the alkali metal or the like, the film mustnot spoil the effect. Thus, although a thin film made of the materialhaving the foregoing heat radiating effect can be used alone, it iseffective to stack the thin film and an insulating film (typically asilicon nitride film (SixNy) or silicon nitride oxide film (SiOxNy)).Note that in the silicon nitride film or silicon nitride oxide film, xand y are respectively arbitrary integers.

A second interlayer insulating film (also referred to as a levelingfilm) is formed on the first passivation film 41, and leveling of a stepdue to the TFT is performed. It is preferable to use an organic resinfilm as the second interlayer insulating film 42, and materials such aspolyimide, polyamide, acrylic, and BCB (benzocyclobutene) may be used.An inorganic film may also be used, of course, provided that it iscapable of sufficient leveling.

Further, reference numeral 43 denotes a pixel electrode made from amaterial having aluminum as its main constituent (aluminum compositionratio between 50 and 99.9%), and projecting portions are formed on itssurface. Reference numeral 44 denotes a cathode made from a metallicfilm containing an alkaline metal or an alkaline earth metal. Thecathode 44 is formed so as to trace the projecting portions of the pixelelectrode 43 at this point, and therefore projecting portions 45 arealso formed in the surface of the cathode 44.

An aluminum film containing from 0.1 to 6.0 weight % (preferably between0.5 and 2.0 weight %) of either silicon (Si), nickel (Ni), or copper(Cu) may be used as the pixel electrode 43.

As the cathode 44, a material having a low work function and containingmagnesium (Mg), lithium (Li), or calcium (Ca) is used. Preferably, anelectrode made of MgAg (material of Mg and Ag mixed at a ratio ofMg:Ag=10:1) is used. In addition, a MgAg/Al electrode, a Li/Alelectrode, and a LIP/Al electrode can be enumerated.

The projecting portions 45 are explained here in detail. An expandedview of a region denoted by reference numeral 204 in FIG. 1 is shown inthe blow up view of FIG. 2. As shown in FIG. 2, taking the spacing(pitch) between the projecting portions 45 as X, it is preferable to setX=0.05 to 1 μm (more preferably between 0.3 and 0.8 μm). In other words,by setting the pitch of the projecting portions 45 to be nearly equal tothe wavelength of visible light, diffuse reflection (irregularreflection) of the reflected light can be made to occur effectively.

Further, when the projecting portions 45 are made into mountain shapesas shown in FIG. 2, it is preferable to set an angle θ formed by a lineparallel to the substrate surface (the surface of the substrate on whichthe thin films are formed) and the projecting portions 45 to θ=30 to 70E (preferably between 50 and 60 E).

In addition, an EL layer 46 is formed on the cathode 44 having theprojecting portions 45. The EL layer 46 is formed by using knownmaterials and structures. Namely, the EL layer may be formed by only alight emitting layer, and it also may be formed using a structurecomprising a hole transporting layer and a light emitting layer, or astructure comprising a hole transporting layer, a light emitting layer,and an electron transporting layer.

Further, the EL layer 46 material may be a low molecular weight materialor a high molecular weight material (polymer). However, it is effectiveto use a high molecular weight material which can be formed by an easyfilm deposition method such as spin coating.

The structure of FIG. 1 is an example of a case of using a monochromaticlight emitting system where one kind of EL element corresponding to anyone of RGB is formed. Although FIG. 2 shows only one pixel, a pluralityof pixels having the same structure are arranged in matrix form in thepixel portion. Note that a well-known material may be adopted for the ELlayer corresponding to any one of RGB.

In addition to the above system, color display can be made by using asystem in which an EL element of white light emission and a color filterare combined, a system in which an EL element of blue or blue-greenlight emission and a fluorescent material (fluorescent color convertinglayer: CCM) are combined, a system in which EL elements corresponding toRGB are stacked, or the like. Of course, it is also possible to makeblack-and-white display by forming an EL layer of white light emissionin a single layer.

An anode 47 made from a transparent conducting film and a secondpassivation film 48 are formed on the EL layer 46. It is possible to usea compound film of indium oxide and tin oxide (referred to as an ITOfilm) or a compound film of indium oxide and zinc oxide as thetransparent conducting film. Tin oxide or zinc oxide may be mixed in ata ratio of 5 to 20% by weight with respect to the indium oxide. Further,the same material as the first passivation layer 41 may also be used asthe second passivation layer 48.

The EL display device of this embodiment includes a pixel having astructure as in FIG. 1, and TFTs having different structures accordingto functions are disposed in the pixel. By this, it is possible to forma switching TFT having a sufficiently low off current value and acurrent controlling TFT strong against hot carrier injection in the samepixel, and it is possible to obtain the EL display device having highreliability and enabling excellent picture display (having highoperation performance).

Embodiment Mode 2

An example of using the present invention in a simple matrix type ELdisplay device is shown in FIG. 16 in embodiment mode 2. In FIG. 16,reference numeral 1601 denotes a substrate, reference numerals 1602 adenote aluminum films with added silicon, and 1602 b are cathodes madefrom lithium fluoride films formed in succession on the aluminum films1602 a. Electrodes 1602 composed of these films in lamination are formedaligned in a stripe shape. The electrodes 1602 are referred to as firstelectrodes here.

In embodiment mode 2, the aluminum films 1602 a are deposited so as tohave projecting portions formed in their surfaces due to steps at thetime of film deposition, and projecting portions 1603 are formed in thesurface of the lithium fluoride film cathodes 1602 b along theprojecting portions formed in the base film aluminum films 1602 a.

An EL layer 1604 is formed by a low molecular weight organic material ora high molecular weight organic material on the electrodes 1602, and aplurality of anodes 1605 made from transparent conducting films areformed on the EL layer 1604. The anodes 1605 are formed perpendicularwith respect to the first electrodes 1602, and are formed aligned in astripe pattern. The electrodes 1605 are referred to as second electrodeshere.

A matrix is thus formed by the first electrodes 1602 and the secondelectrodes 1605, and EL elements are formed at intersecting portions bythe first electrodes (cathodes), the EL layer, and the second electrodes(anodes). A predetermined voltage is then applied to the firstelectrodes 1602 and the second electrodes 1605, and the EL layer 1604 ismade to emit light.

In portions which do not emit light, the surface of the cathodes 1602 bis visible at this point, but external light is reflected diffusely(irregularly) by the projecting portions 1603, and therefore the face ofan observer and scenery is not reflected. In other words, it is notnecessary to use an elliptical film or the like, and therefore it ispossible to reduce the manufacturing cost of the EL display device.

Embodiment 1

The embodiments of the present invention are explained using FIGS. 4A to6C. A method of simultaneous manufacture of a pixel portion, and TFTs ofa driver circuit portion formed in the periphery of the pixel portion,is explained here. Note that in order to simplify the explanation, aCMOS circuit is shown as a basic circuit for the driver circuits.

First, as shown in FIG. 4A, a base film 301 is formed with a 300 nmthickness on a glass substrate 300. Oxidized silicon nitride films arelaminated as the base film 301 in embodiment 1. It is good to set thenitrogen concentration at between 10 and 25 wt % in the film contactingthe glass substrate 300.

Besides, as a part of the under film 301, it is effective to provide aninsulating film made of a material similar to the first passivation film41 shown in FIG. 2. The current controlling TFT is apt to generate heatsince a large current is made to flow, and it is effective to provide aninsulating film having a heat radiating effect at a place as close aspossible.

Next, an amorphous silicon film (not shown in the figures) is formedwith a thickness of 50 nm on the base film 301 by a known depositionmethod. Note that it is not necessary to limit this to the amorphoussilicon film, and another film may be formed provided that it is asemiconductor film containing an amorphous structure (including amicrocrystalline semiconductor film). In addition, a compoundsemiconductor film containing an amorphous structure, such as anamorphous silicon germanium film, may also be used. Further, the filmthickness may be made from 20 to 100 nm.

The amorphous silicon film is then crystallized by a known method,forming a crystalline silicon film (also referred to as apolycrystalline silicon film or a polysilicon film) 302. Thermalcrystallization using an electric furnace, laser annealingcrystallization using a laser, and lamp annealing crystallization usingan infrared lamp exist as known crystallization methods. Crystallizationis performed in embodiment 1 using light from an excimer laser whichuses XeCl gas.

Note that pulse emission type excimer laser light formed into a linearshape is used in embodiment 1, but a rectangular shape may also be used,and continuous emission argon laser light and continuous emissionexcimer laser light can also be used.

In this embodiment, although the crystalline silicon film is used as theactive layer of the TFT, it is also possible to use an amorphous siliconfilm. However, in order to increase an opening rate of a pixel by makingan area of the current controlling TFT as small as possible, it isadvantageous to use the crystalline silicon film through which a currentcan easily flow.

Note that it is effective to form the active layer of the switching TFT,in which there is a necessity to reduce the off current, by theamorphous silicon film, and to form the active layer of the currentcontrol TFT by the crystalline silicon film. Electric current flows withdifficulty in the amorphous silicon film because the carrier mobility islow, and the off current does not easily flow. In other words, the mostcan be made of the advantages of both the amorphous silicon film,through which current does not flow easily, and the crystalline siliconfilm, through which current easily flows.

Next, as shown in FIG. 4B, a protecting film 303 is formed on thecrystalline silicon film 302 with a silicon oxide film having athickness of 130 nm. This thickness may be chosen within the range of100 to 200 nm (preferably between 130 and 170 nm). Furthermore, otherfilms may also be used providing that they are insulating filmscontaining silicon. The protecting film 303 is formed so that thecrystalline silicon film is not directly exposed to plasma duringaddition of an impurity, and so that it is possible to have delicateconcentration control of the impurity.

Resist masks 304 a and 304 b are then formed on the protecting film 303,and an impurity element which imparts n-type conductivity (hereafterreferred to as an n-type impurity element) is added. Note that elementsresiding in periodic table group 15 are generally used as the n-typeimpurity element, and typically phosphorus or arsenic can be used. Notethat a plasma doping method is used, in which phosphine (PH₃) is plasmaactivated without separation of mass, and phosphorus is added at aconcentration of 1×10¹⁸ atoms/cm³ in embodiment 1. An ion implantationmethod, in which separation of mass is performed, may also be used, ofcourse.

The dose amount is regulated so that the n-type impurity element iscontained in n-type impurity regions 305 and 306, thus formed by thisprocess, at a concentration of 2×10¹⁹ to 5×10¹⁹ atoms/cm³ (typicallybetween 5×10¹⁷ and 5×10¹⁹ atoms/cm³).

Next, as shown in FIG. 4C, the protecting film 303 is removed, and anactivation of the added periodic table group 15 elements is performed. Aknown technique of activation may be used as the means of activation,and activation is done in embodiment 1 by irradiation of excimer laserlight. A pulse emission type excimer laser and a continuous emissiontype excimer laser may both, of course, be used, and it is not necessaryto place any limits on the use of excimer laser light. The goal is theactivation of the added impurity element, and it is preferable thatirradiation is performed at an energy level at which the crystallinesilicon film does not melt. Note that the laser irradiation may also beperformed with the protecting film 303 in place.

The activation by heat treatment may also be performed along withactivation of the impurity element by laser light. When activation isperformed by heat treatment, considering the heat resistance of thesubstrate, it is good to perform heat treatment on the order of 450 to550° C.

Boundary portions of the n-type impurity regions 305 and 306, that is,boundary portions (connecting portions) thereof with regions which arepresent in the periphery of the n-type impurity regions 305 and 306 andare not added with the n-type impurity are delineated by this process.This means that, at the point when the TFTs are later completed,extremely good connections can be formed between LDD regions and channelforming regions.

Unnecessary portions of the crystalline silicon film are removed next,as shown in FIG. 4D, and island shape semiconductor films (hereafterreferred to as active layers) 307 to 310 are formed.

Then, as shown in FIG. 4E, a gate insulating film 311 is formed,covering the active layers 307 to 310. An insulating film containingsilicon and with a thickness of 10 to 200 nm, preferably between 50 and150 nm, may be used as the gate insulating film 311. A single layerstructure or a lamination structure may be used. A 110 nm thick oxidizedsilicon nitride film is used in embodiment 1.

A conducting film is formed next with a thickness of 200 to 400 nm, andis patterned, forming gate electrodes 312 to 316. Single layerconducting films may be formed for the gate electrodes 312 to 316, andwhen necessary, it is preferable to form a lamination film such of twolayers or three layers. All known conducting films can be used as thegate electrode material.

Typically, it is possible to use a film made of an element selected fromtantalum (Ta), titanium (Ti), molybdenum (No), tungsten (W), chromium(Cr), and silicon (Si), a film of nitride of the above element(typically a tantalum nitride film, tungsten nitride film, or titaniumnitride film), an alloy film of combination of the above elements(typically Mo—W alloy, Mo—Ta alloy), or a silicide film of the aboveelement (typically a tungsten silicide film, titanium silicide film). Ofcourse, the films may be used as a single layer or a laminate layer.

In this embodiment, a laminate film of a tungsten nitride (WN) filmhaving a thickness of 50 nm and a tungsten (W) film having a thicknessof 350 nm is used. These may be formed by a sputtering method. When aninert gas of Xe, Ne or the like is added as a sputtering gas, filmpeeling due to stress can be prevented.

The gate electrodes 313 and 316 are formed at this time so as to overlapa portion of the n-type impurity regions 305 and 306, respectively,sandwiching the gate insulating film 311. This overlapping portion laterbecomes an LDD region overlapping the gate electrode.

Next, an n-type impurity element (phosphorous is used in embodiment 1)is added in a self-aligning manner with the gate electrodes 312 to 316as masks, as shown in FIG. 5A. The addition is regulated so thatphosphorus is added to impurity regions 317 to 323 thus formed at aconcentration of 1/10 to ½ that of the impurity regions 305 and 306(typically between ¼ and ⅓). Specifically, a concentration of 1×10¹⁶ to5×10¹⁸ atoms/cm³ (typically 3×10¹⁷ to 3×10¹⁸ atoms/cm³) is preferable.

Resist masks 324 a to 324 d are formed next, with a shape covering thegate electrodes etc., as shown in FIG. 5B, and an n-type impurityelement (phosphorus is used in embodiment 1) is added, forming impurityregions 325 to 331 containing a high concentration of phosphorus. Iondoping using phosphine (PH₃) is also performed here, and is regulated sothat the phosphorus concentration of these regions is from 1×10²⁰ to1×10²¹ atoms/cm³ (typically between 2×10²⁰ and 5×10²¹ atoms/cm³).

A source region or a drain region of the n-channel TFT is formed by thisprocess, and in the switching TFT, a portion of the n-type impurityregions 320 to 322 formed by the process of FIG. 5A remains. Theseremaining regions correspond to the LDD regions 15 a to 15 d of theswitching TFT in FIG. 1.

Next, as shown in FIG. 5C, the resist masks 324 a to 324 d are removed,and a new resist mask 332 is formed. A p-type impurity element (boron isused in embodiment 1) is then added, forming impurity regions 333 and334 containing a high concentration of boron. Boron is added here toform impurity regions 333 and 334 at a concentration of 3×10²⁰ to 3×10²¹atoms/cm³ (typically between 5×10²⁰ and 1×10²¹ atoms/cm³) by ion dopingusing diborane (B₂H₆).

Note that phosphorus has already been added to the impurity regions 333and 334 at a concentration of 1×10²⁰ to 1×10²¹ atoms/cm³, but boron isadded here at a concentration of at least 3 times of the phosphorus.Therefore, the n-type impurity regions already formed completely invertto p-type, and function as p-type impurity regions.

Next, after removing the resist mask 332, an insulating film (protectingfilm) 335 used for protecting the gate is formed. The insulating film335 is formed in order to prevent an increase in resistance value of thegate electrode due to oxidation during the heat treatment which isperformed next. A 50 to 300 nm (preferably between 100 and 200 nm) thickinsulating film containing silicon may be formed as the insulating film335. (See FIG. 5D.)

The n-type and p-type impurity elements added to the active layer atvarious concentrations are activated next. Furnace annealing, laserannealing, lamp annealing, or a combination of these processes can beused as a means of activation. In embodiment 1, heat treatment (furnaceannealing) is performed for 4 hours at 550 EC in a nitrogen atmospherein an electric furnace.

A first interlayer insulating film 336 is formed next, as shown in FIG.6A. A single layer insulating film containing silicon is used as thefirst interlayer insulating film 336, while a lamination film may becombined in between. Further, a film thickness of between 400 nm and 1.5μm may be used. A lamination structure of an 800 nm thick silicon oxidefilm on a 200 nm thick oxidized silicon nitride film is used inembodiment 1.

In addition, heat treatment is performed for 1 to 12 hours at 300 to450° C. in an environment containing between 3 and 100% hydrogen,performing hydrogenation. This process is one of hydrogen termination ofdangling bonds in the semiconductor film by hydrogen which is thermallyactivated. Plasma hydrogenation (using hydrogen activated by a plasma)may also be performed as another means of hydrogenation.

Note that the hydrogenation step may also be inserted during theformation of the first interlayer insulating film 336. Namely, hydrogenprocessing may be performed as above after forming the 200 nm thickoxidized silicon nitride film, and then the remaining 800 nm thicksilicon oxide film may be formed.

Next, a contact hole is formed in the first interlayer insulating film336, and source wiring lines 337 to 340 and drain wiring lines 341 to343 are formed. In this embodiment, this electrode is made of a laminatefilm of three-layer structure in which a titanium film having athickness of 100 nm, an aluminum film containing titanium and having athickness of 300 nm, and a titanium film having a thickness of 150 nmare continuously formed by a sputtering method. Of course, otherconductive films may be used.

A first passivation film 344 is formed next with a thickness of 50 to500 nm (typically between 200 and 300 nm). A 300 nm thick oxidizedsilicon nitride film is used as the first passivation film 344 inembodiment 1. This may also be substituted by a silicon nitride film. Itis of course possible to use the same materials as those of the firstpassivation film 41 of FIG. 1.

Note that it is effective to perform plasma processing using a gascontaining hydrogen such as H₂ or NH₃ etc. before the formation of theoxidized silicon nitride film. Hydrogen activated by this preprocess issupplied to the first interlayer insulating film 336, and the filmquality of the first passivation film 344 is improved by performing heattreatment. At the same time, the hydrogen added to the first interlayerinsulating film 336 diffuses to the lower side, and the active layerscan be hydrogenated effectively.

Next, as shown in FIG. 6B, a second interlayer insulating film 345 madeof organic resin is formed. As the organic resin, it is possible to usepolyimide, polyamide, acryl, BCB (benzocyclobutene) or the like.Especially, since the second interlayer insulating film 345 is primarilyused for flattening, acryl excellent in flattening properties ispreferable. In this embodiment, an acrylic film is formed to a thicknesssufficient to flatten a stepped portion formed by TFTs. It isappropriate that the thickness is preferably made 1 to 5 μm (morepreferably, 2 to 4 μm).

Next, the second interlayer insulating film 345 and the firstpassivation film 344 are etched, forming a contact hole which reachesthe drain wiring 343, and a pixel electrode 346 is formed. An aluminumfilm containing 1 wt % Si is used as the pixel electrode 346 inembodiment 1. An aluminum film having projecting portions in its surfaceis formed by depositing the aluminum film by sputtering at a substratetemperature of 50 to 200 EC (preferably between 70 and 150 EC). Notethat between 0.1 and 5% moisture may also be added to the sputteringgas.

The pixel electrode 346 having projecting portions in its surface canthus be formed. The pattern of projecting portions formed is irregularin this case, but the aim is diffuse reflection (irregular reflection)of light, and therefore irregularity does not become a problem inparticular.

If it is necessary to form regular projecting portions, then the surfaceof the pixel electrode is patterned and then the projecting portions areformed, or a means of performing patterning the surface of the secondinterlayer insulating film 345, forming the projecting portions, andthen forming the pixel electrode on the projecting portions may beemployed. Further, when using a material capable of selective etching byutilizing orienting characteristics as the pixel electrode 346 material,the projecting portions can easily be obtained by performing surfaceprocessing by using an etchant so as to expose a specifically orientedsurface. Techniques such as a technique of pit formation of a siliconsurface are known as typical techniques of selective etching.

A cathode 347 made from a MgAg electrode is formed next with a thicknessof 120 nm. The film thickness may be from 80 to 200 nm (typicallybetween 100 and 150 nm). Further, as shown in embodiment mode 1, aLiF/Al electrode (a lamination film of a lithium fluoride film and analuminum film) may also be used. In any case, it is preferable to use amaterial having a small work function.

The cathode 347 is formed along the projecting portions formed in thesurface of the pixel electrode 346 at this time, and therefore thecathode 347 is also formed having projecting portions in its surface.The problem of an observer's face being reflected in the displayportion, as shown in the conventional example, is a problem ofreflection on the cathode surface, and by forming the projectingportions in the cathode surface and generating diffuse reflection(irregular reflection), this type of inconvenience can be prevented.

An EL layer 348 is formed next by evaporation. A two layer structure ofa hole transporting layer and an emitting layer is used as the EL layerin embodiment 1 (shown as a single layer in the drawings), but there arealso cases of forming a hole injecting layer, an electron injectinglayer, or an electron transporting layer. Many examples of this type ofcombination have already been reported upon, and any of theseconstitutions may also be used.

Furthermore, moisture adhering to the interface of the EL layer 348 andthe cathode 347, particularly oxygen, must be avoided completely. Thisis because the EL layer 348 oxidizes easily and deteriorates. Thecathode 347 and the EL layer 348 are therefore formed successively byusing evaporation without breaking the vacuum. Specifically, atris-(8-quinolinolate) aluminum (referred to as Alq) is formed firstwith a thickness of 50 nm as the emitting layer, and a 70 nm thick TPD(triphenylamine derivative) is formed, on the emitting layer as the holetransporting layer. The two layer structure EL layer 348 is thus formed.

Note that an example of forming the EL layer using low molecular weightorganic materials is shown in embodiment 1, but high molecular weightorganic materials may also be used, and a combination of both may alsobe used. Further, any known structure (a single layer structure or alamination structure) may also be used as the EL layer structure.

The structure of FIG. 6B is thus obtained. The EL layer 348 is exposedin this state, and therefore it is important to place the substrate inan atmosphere filled by an inert gas such as nitrogen or a noble gas.The substrate is then conveyed to a sputtering apparatus withoutexposure to the atmosphere, and anodes 349 are formed from a transparentconducting film. The film thickness may be set from 100 to 200 nm.

Generally known materials such as ITO (an indium oxide and tin oxidecompound) or an indium oxide and zinc oxide compound can be used as thetransparent conducting film. Potassium may also be added to the indiumoxide and zinc oxide compound.

In addition, a second passivation film 350 made from an insulating filmcontaining silicon is formed on the anodes 349 in embodiment 1. Thesecond passivation film 350 is also preferably formed in successionwithout breaking the vacuum. A 300 nm thick silicon nitride film isformed as the second passivation film 350 in embodiment 1.

In this way, an active matrix type EL display device having a structureas shown in FIG. 6C is completed. In the active matrix type EL displaydevice of this embodiment, a TFT having an optimum structure is disposedin not only the pixel portion but also the driving circuit portion, sothat very high reliability is obtained and operation characteristics canalso be improved.

First, a TFT having a structure to decrease hot carrier injection so asnot to drop the operation speed thereof as much as possible is used asan n-channel TFT 205 of a CMOS circuit forming a driving circuit. Notethat the driving circuit here includes a shift register, a buffer, alevel shifter, a sampling circuit (sample and hold circuit) and thelike. In the case where digital driving is made, a signal conversioncircuit such as a D/A converter can also be included.

In the case of this embodiment, as shown in FIG. 6C, the active layer ofthe n-channel 205 includes a source region 355, a drain region 356, anLDD region 357 and a channel formation region 358, and the LDD region357 overlaps with the gate electrode 313, putting the gate insulatingfilm 311 therebetween.

Consideration not to drop the operation speed is the reason why the LDDregion is formed at only the drain region side. In this n-channel TFT205, it is not necessary to pay attention to an off current value verymuch, rather, it is better to give importance to an operation speed.Thus, it is desirable that the LDD region 357 is made to completelyoverlap with the gate electrode to decrease a resistance component to aminimum. That is, it is preferable to remove the so-called offset.

Further, an active layer of a p-channel TFT 206 of a CMOS circuitincludes a source region 359, a drain region 360, and a channel formingregion 361, and an LDD region is not formed in particular. Deteriorationdue to hot carrier injection does not become much of a problem for thep-channel TFT even with this structure, but it is also possible to makea countermeasure against hot carriers by forming an LDD region similarto that of the n-channel TFT 205.

Note that, among the driving circuits, the sampling circuit is somewhatunique compared to the other sampling circuits, in that a large electriccurrent flows in both directions in the channel forming region. Namely,the roles of the source region and the drain region are interchanged. Inaddition, it is necessary to control the value of the off current to beas small as possible, and with that in mind, it is preferable to use aTFT having functions which are on an intermediate level between theswitching TFT and the current control TFT in the sampling circuit. Acombination of an n-channel TFT 207 and a p-channel TFT 208 as shown inFIG. 10 is used as the sampling circuit in embodiment 1.

A portion of LDD regions 801 a and 801 b overlap a gate electrode 803through a gate insulating film 802, as shown in FIG. 10, in then-channel TFT 207 which forms the sampling circuit. This effect is thesame as that stated by the explanation of the current control TFT 202,and for the case of the sampling circuit, the fact that the LDD regions801 a and 801 b are formed with a shape sandwiching a channel formingregion 804 is a point of difference.

Actually, when the state of FIG. 6C is completed, it is preferable tomake packaging (sealing) by a housing member such as a protection filmhaving high airtightness and less degassing (laminate film, ultravioletray curing resin film, etc.) or a sealing material so as to preventexposure to the outer air. At that time, when the inside of the sealingmember is made an inert gas atmosphere, or a moisture absorbent (forexample, barium oxide) is disposed in the inside, the reliability(lifetime) of the EL layer is improved.

After the airtightness is raised by processing such as packaging, aconnector (flexible print circuit: FPC) for connecting a terminalextended from the element or circuit formed on the substrate to anexternal signal terminal is attached so that a product is completed. Inthe present specification, the EL display device, which is made to havesuch a state that it can be shipped, is called an EL module.

Here, the structure of the active matrix type EL display device of thisembodiment will be described with reference to a perspective view ofFIG. 7. The active matrix type EL display device of this embodiment isconstituted by a pixel portion 602, a gate side driving circuit 603, anda source side driving circuit 604 formed on a glass substrate 601. Aswitching TFT 605 of a pixel portion is an n-channel TFT, and isdisposed at an intersection point of a gate wiring line 606 connected tothe gate side driving circuit 603 and a source wiring line 607 connectedto the source side driving circuit 604. The drain of the switching TFT605 is connected to the gate of a current controlling TFT 608.

In addition, the source side of the current control TFT 608 is connectedto a power supply line 609. With the structure of this embodiment, thepower supply line 609 is connected to the current control TFT 608, and adrain of the current control TFT 608 is connected to an EL element 610.

If the current control TFT 608 is an n-channel TFT, then a cathode ofthe EL element 610 is electrically connected to the drain. Further, fora case of using a p-channel TFT for the current control TFT 608, ananode of the EL element 610 is electrically connected to the drain.

Input wiring lines (connection wiring lines) 612 and 613 fortransmitting signals to the driving circuits and an input wiring line614 connected to the current supply line 609 are provided in an FPC 611as an external input-output terminal.

An example of circuit structure of the EL display device shown in FIG. 7is shown in FIG. 8. The EL display device of this embodiment includes asource side driving circuit 701, a gate side driving circuit (A) 707, agate side driving circuit (B) 711, and a pixel portion 706. Note that inthe present specification, the term driving circuit is a general termincluding the source side driving circuit and the gate side drivingcircuit.

The source side driving circuit 701 is provided with a shift register702, a level shifter 703, a buffer 704, and a sampling circuit (sampleand hold circuit) 705. The gate side driving circuit (A) 707 is providedwith a shift register 708, a level shifter 709, and a buffer 710. Thegate side driving circuit (B) 711 also has the same structure.

Here, the shift registers 702 and 708 have driving voltages of 5 to 16 V(typically 10 V) respectively, and the structure indicated by 205 inFIG. 6C is suitable for an n-channel TFT used in a CMOS circuit formingthe circuit.

Besides, for each of the level shifters 703 and 709 and the buffers 704and 710, similarly to the shift register, the CMOS circuit including then-channel TFT 205 of FIG. 6C is suitable. Note that it is effective tomake a gate wiring line a multi-gate structure such as a double gatestructure or a triple gate structure in improving of reliability of eachcircuit.

Besides, since the source region and drain region are inverted and it isnecessary to decrease an off current value, a CMOS circuit including then-channel TFT 207 of FIG. 10 is suitable for the sampling circuit 705.

In the pixel portion 706 are disposed pixels having the structure shownin FIG. 1.

The foregoing structure can be easily realized by manufacturing TFTs inaccordance with the manufacturing steps shown in FIGS. 4 to 6. In thisembodiment, although only the structure of the pixel portion and thedriving circuit is shown, if the manufacturing steps of this embodimentare used, it is possible to form a logical circuit other than thedriving circuit, such as a signal dividing circuit, a D/A convertercircuit, an operational amplifier circuit, γ-correction circuit, or thelike on the same substrate, and further, it is believed that a memoryportion, a microprocessor, or the like can be formed.

Further, an EL module of this embodiment including a housing member aswell will be described with reference to FIGS. 11A and 11B. Note that asnecessary, reference numbers used in FIGS. 7 and 8 will be quoted.

A pixel portion 1101, a source side driving circuit 1102, and a gateside driving circuit 1103 are formed on a substrate (including an underfilm below a TFT) 1100. Various wiring lines from the respective drivingcircuits lead to an FPC 611 through input-output wiring lines 612 to 614and are connected to an external equipment.

A sealing material 1104 is formed at this time so as to surround atleast the pixel portion, and preferably the driver circuits and thepixel portion. Note that a plate shape material possessing a concaveportion so as to surround the element portion may also be used as thesealing material 1104, and that a sheet shape ultraviolet hardened resinmay also be used. When using a metallic plate possessing a concaveportion so as to surround the element portion as the sealing material1104, the sealing material 1104 is fixed to the substrate 1100 by anadhesive 1105, forming an airtight space between the sealing material1104 and the substrate 1100. The EL element is in a state of beingcompletely enclosed in the airtight space at this point, and iscompletely cutoff from the atmosphere.

A plate shape material such as amorphous glass (such as borosilicateglass and quartz), crystallized glass, and ceramic glass can be used asthe sealing material 1104, and an organic resin (such as an acrylicresin, a styrene resin, a polycarbonate resin, or an epoxy resin) and asilicone resin can also be used. Whichever is used, the sealing material1104 must be transparent when manufacturing an EL display device typehaving a substrate which outputs light in the reflection side, as inembodiment 1.

As a material of the adhesive 1105, an adhesive of epoxy resin, acrylateresin, or the like can be used. Further, thermosetting resin orphoto-curing resin can also be used as the adhesive. However, it isnecessary to use such material as to block penetration of oxygen andmoisture to the utmost.

In addition, a gap 1106 between the sealing material and the substrate1100 is preferably filled with an inert gas (such as argon, helium, ornitrogen). Further, this is not limited to a gas, and it is alsopossible to use a transparent inert liquid. It is also effective to forma drying agent in the gap 1106. Materials such as those disclosed inJapanese Patent Application Laid-open No. 9-148066 can be used as thedrying agent. Barium oxide may typically be used.

Furthermore, a plurality of pixels are formed in the pixel region havingthe respective isolated EL elements, as shown in FIG. 11B, and all ofthem have an anode 1107 as a common electrode. The cathodes and the ELlayer may be formed only in the pixel portion at this point; it is notnecessary to form them on the driver circuits. Of course, there is noproblem in forming them on the driver circuits, but considering thatalkaline metals are included in the EL layer, it is preferable to notform them on the driver circuits. Note that the EL layer is weak withrespect to moisture and cannot be patterned, and therefore it may beformed selectively by evaporation using a shadow mask.

Note also that the anode 1107 is connected to an input-output wiring1109 in a region denoted by reference numeral 1108. The input-outputwiring 1109 is a power supply line for imparting a fixed voltage (aground voltage, specifically 0 V, in embodiment 1) to the anode 1107,and it is electrically connected to an FPC 611 through a conductingpaste material 1110.

A manufacturing process for realizing a contact structure in the region1108 is explained here using FIGS. 12A to 12C.

First, in accordance with the steps of this embodiment, the state ofFIG. 6A is obtained. At this time, at an end portion of the substrate(region indicated by 1108 in FIG. 11B), the first interlayer insulatingfilm 336 and the gate insulating film 311 are removed, and aninput-output wiring line 1109 is formed thereon. Of course, it is formedat the same time as the source wiring line and the drain wiring line ofFIG. 6A (FIG. 12A).

Next, when etching the second interlayer insulating film 345 and thefirst passivation film 344 in FIG. 6B, a region denoted by referencenumeral 1201 is removed, and an opening portion 1202 is formed. (FIG.12B.)

A process of forming the EL element (a process of forming the pixelelectrode, the EL layer, and the cathode) is performed in the pixelportion in this state. A mask material is used so that the cathode 347and the EL layer 348 are not formed in the region shown in FIGS. 12A to12C. After then forming the EL layer 348, the anode 349 is formed. Theanode 349 and the input-output wiring 1109 are thus electricallyconnected. In addition, the state of FIG. 12C is obtained by forming thesecond passivation film 350.

Through the foregoing steps, the contact structure of the regionindicated by 1108 of FIG. 11B is realized. The input-output wiring line1109 is electrically connected to the FPC 611 through a gap between thehousing member 1104 and the substrate 1100 (however, the gap is filledwith the adhesive 1105). Note that although the description has beenmade here on the input wiring line 1109, other output wiring lines 612to 614 are also connected to the FPC 611 through the portion under thehousing member 1104 in the same manner.

Embodiment 2

In this embodiment, an example in which a structure of a pixel is madedifferent from the structure shown in FIG. 3B will be described withreference to FIG. 13.

The two pixels shown in FIG. 3B are arranged to become symmetrical withrespect to the power supply line 211 which imparts a ground electricpotential. In other words, by sharing the power supply line 212 betweentwo pixels, as shown in FIG. 13, the number of necessary wirings can bereduced. Note that structures such as the TFT structures placed withinthe pixels remain as is.

If such structure is adopted, it becomes possible to manufacture a moreminute pixel portion, and the quality of an image is improved.

Note that the structure of this embodiment can be easily realized inaccordance with the manufacturing steps of the embodiment 1, and withrespect to the TFT structure or the like, the description of theembodiment 1 or FIG. 2 may be referred to.

Embodiment 3

Cases of using top gate type TFTs were explained by embodiment 1 andembodiment 2, but the present invention is not limited to a TFTstructure, and it may also be implemented using a bottom gate type TFT(typically a reverse stagger type TFT). Further, the reverse staggertype TFT may be formed by any means.

The reverse stagger type TFT is a good structure having fewer processesthan the top gate type TFT, and it is therefore extremely advantageousin lowering manufacturing costs, an object of the present invention.

Embodiment 4

In the EL display devices explained by embodiment mode 1 and embodiment1, by giving the switching TFTs in the pixels a multi-gate structure,the value of the off current of the switching TFT is reduced, and thenecessity of a storage capacitor is eliminated. This is a design foreffectively utilizing the exclusive surface area of the storagecapacitor as a light emitting region.

However, even without completely eliminating the storage capacitor, bymaking its exclusive surface area smaller, an effect of enlarging thelight emitting surface area can be obtained. Namely, it is sufficient toreduce the value of the off current and to shrink the size of theexclusive surface area of the storage capacitor by making the switchingTFT into a multi-gate structure.

In this case a storage capacitor 1401 may also be formed with respect tothe switching TFT 201, in parallel with the gate of the current controlTFT 202, as shown in FIG. 14.

Note that the constitution of embodiment 4 can be freely combined withthe constitutions of any one of embodiments 1 to 3. Namely, a storagecapacitor is provided in the pixel and there is no limit on the TFTstructure or EL layer materials, etc.

Embodiment 5

Laser crystallization is used as the means of forming the crystallinesilicon film 302 in embodiment 1, but a case of using a different meansof crystallization is explained in embodiment 5.

Crystallization is performed in embodiment 5 by using the techniquerecorded in Japanese Patent Application Laid-open No. 7-130652 afterforming an amorphous silicon film. The technique recorded in the abovepatent application is one of obtaining a crystalline silicon film havinggood crystallinity by using an element such as nickel as a catalyst forpromoting crystallization.

Further, after completing the crystallization process, a process ofremoving the catalyst used in crystallization may also be performed. Inthis case, the catalyst may be gettered by the technique recorded inJapanese Patent Application Laid-open No. 10-270363 or in JapanesePatent Application Laid-open No. 8-330602.

Furthermore, the TFT may also be formed by using the technique recordedin Japanese Patent Application Laid-open No. 11-076967 by the applicantof the present invention.

The manufacturing process shown in embodiment 1 is thus one exemplary,and provided that the structures shown in FIG. 1, FIG. 2, or in FIG. 6Cof embodiment 1 can be realized, then other manufacturing processes mayalso be used without problems.

Note that it is possible to freely combine the constitution ofembodiment 5 with the constitutions of any one of embodiments 1 to 4.

Embodiment 6

Analog driving using an analog signal as a pixel signal can be performedwhen driving the EL display device of the present invention, and digitaldriving using a digital signal can also be performed.

When performing analog driving, an analog signal is sent to a sourcewiring line of a switching TFT, and the analog signal containinggradation information becomes a gate voltage of a current control TFT.The current flowing in an EL element is then controlled by the currentcontrol TFT, and gradation display is performed by controlling thestrength of the light emitted by the EL element.

When performing digital driving, on the other hand, gradation displayreferred to as time partitioned driving is performed, differing fromanalog gradation display. Namely, by regulating the length of time oflight emission, color gradations are shown to be changing visually.

The response speed of the EL element is extremely fast compared withthat of a liquid crystal element, and it is possible to drive it at highspeed. It can therefore be said that the EL element is suitable for timepartition driving in which one frame is partitioned into a plurality ofsubframes and then gradation display is performed.

The present invention is thus a technique related to element structures,and therefore any driving method may be used.

Embodiment 7

An example of using an organic EL material as an EL layer is shown inembodiment 1, but the present invention can also be implemented using aninorganic EL material. However, present inorganic EL materials haveextremely high driving voltages, and therefore a TFT having voltageresistance characteristics which can withstand the high driving voltagesmust be used when performing analog driving.

Alternatively, if an inorganic EL material having a lower drivingvoltage is developed in the future, it will be possible to apply this tothe present invention.

Furthermore, it is possible to freely combine the constitution ofembodiment 7 with the constitutions of any of embodiments 1 to 6.

Embodiment 8

An example of forming an EL element using the thin film formingapparatus shown in FIG. 15 is shown in embodiment 8. In FIG. 15,reference numeral 901 denotes a conveyor chamber for performinginsertion or extraction of a substrate, and is also referred to as aload-lock chamber. In embodiment 8, a substrate, on which processing isperformed in accordance with the steps of embodiment 1 up to theformation of the pixel electrode 346 of FIG. 6B, is first set into acarrier 902. Note that the conveyor chamber 901 may also be separatedinto a substrate insertion chamber and a substrate extraction chamber.

Reference numeral 903 denotes a common chamber containing a mechanismfor conveying the substrate (hereafter referred to as a conveyormechanism). A plurality of processing chambers (denoted by referencenumerals 906 to 910) are connected to the common chamber 903 throughgates 905 a to 905 f.

In order to completely seal off each of the processing chambers from thecommon chamber 903 by the gates 905 a to 905 f, airtight seals areobtained. It therefore becomes possible to perform processing under avacuum by installing an evacuation pump in each of the processingchambers. It is possible to use a rotary oil pump, a mechanical boosterpump, a turbo molecular pump, or a cryopump as the evacuation pump, butit is preferable to use the cryopump which is effective in removingmoisture.

The substrate is then transported to the common chamber 903 by theconveyor mechanism 904, and is next transported to a first gas phasefilm deposition processing chamber 906. Cathode formation by evaporationor sputtering is performed in the first gas phase film depositionprocessing chamber 906. A MgAg alloy in which magnesium and silver areevaporated together at a ratio of 10:1 is used as the cathode materialin embodiment 8.

Next, the substrate is transported from the first gas phase filmdeposition processing chamber 906 to a solution application processingchamber 907. A solution containing an EL material is applied by spincoating in the liquid application processing chamber 907, forming apolymer precursor containing a high molecular weight (polymer) ELmaterial. A solution of polyvinylcarbazole dissolved in chloroform isused as the solution containing the EL material in embodiment 8. Ofcourse, other high molecular weight EL materials (typically materialssuch as polyphenylene vinylene or polycarbonate) or other organicsolvents (typically solvents such as dichloromethane or tetrahydrofuran)may also be combined.

The substrate is then transported from the solution applicationprocessing chamber 907 to a firing chamber 908. The EL material ispolymerized by firing (heat treatment) in the firing chamber 908. Heattreatment is performed in embodiment 8 at a temperature of 50 to 150° C.(preferably between 110 and 120° C.) with respect to the entiresubstrate by heating the stage with a heater. Excess chloroform is thusvaporized and the high molecular weight light emitting layer made frompolyvinylcarbazole is formed. This single layer light emitting layer isused as the EL layer in embodiment 8.

The substrate is next transported from the firing chamber 908 to asecond gas phase film deposition processing chamber 909. An anode madefrom transparent conducting film is formed on the high molecular weightlight emitting layer (EL layer) in the second gas phase film depositionprocessing chamber 909. A compound of 10 to 15% zinc oxide mixed intoindium oxide is used in embodiment 8.

Next, the substrate is conveyed from the second gas phase filmdeposition processing chamber 909 to a third gas phase film depositionprocessing chamber 910. A passivation film made from an insulating film,preferably an insulating film containing silicon, is formed in the thirdgas phase film deposition processing chamber 910. The passivation layeris formed in order to protect the EL layer from moisture and oxygen.

The substrate is then conveyed from the third gas phase film depositionprocessing chamber 910 to the carrier 902 placed in the conveyor chamber901. The series processing using the thin film formation apparatus ofFIG. 15 is thus completed.

The advantage of using the thin film formation apparatus shown in FIG.15 is that processing can be performed in succession from the formationof the cathode to the formation of the passivation layer, without thesubstrate once being exposed to the atmosphere (in particular,moisture). In other words, all processing is performed under a vacuum orunder a dry inert gas atmosphere, and therefore degradation of the lightemitting layer is avoided.

In addition, a processing chamber for preforming spin coating is alsoinstalled in the same thin film formation apparatus, and therefore it ispossible to form the EL element using a high molecular weight ELmaterial. When forming the EL layer by evaporation or sputtering, a gasphase film deposition processing chamber may of course be installed as asubstitute for the solution application processing chamber and thefiring chamber.

Note that the thin film formation apparatus shown in embodiment 8 can beused when forming the EL element in the manufacturing process ofembodiment 1. Therefore, it is also possible to use the thin filmformation apparatus of embodiment 8 to obtain the structures shown inembodiments 2 to 7 using the manufacturing processes of embodiment 1.

Embodiment 9

An active matrix type EL display device (EL module) formed byimplementing the present invention has superior visibility in brightlocations compared with liquid crystal display device because the ELdisplay device is a self-emitting type. Its use as a direct view ELdisplay device (indicating a display incorporating the EL module) aretherefore wide.

Note that one advantage of the EL display over the liquid crystaldisplay that can be given is its wide viewing angle. The EL display ofthe present invention may therefore be used as a display (displaymonitor) having a diagonal size equal to or greater than 30 inches(typically equal to or greater than 40 inches) in appreciatingbroadcasts such as TV broadcasts on a large size screen.

Further, the present invention can be used not only as an EL display(such as in a personal computer monitor, a TV broadcast receivingmonitor, or an advertisement display monitor), but can also be used as adisplay for various electronic devices.

The following can be given as examples of such electronic devices: avideo camera; a digital camera; a goggle type display (head mounteddisplay); a game machine; a car navigation system; a personal computer;a portable information terminal (such as a mobile computer, a portabletelephone, or an electronic book); and an image playback devicefurnished with a recording medium (specifically, a device furnished witha display which can play back and display recording mediums such as acompact disk (CD), a laser disk (LD), or a digital video disk (DVD)).Examples of these electronic devices are shown in FIGS. 17A to 17F.

FIG. 17A is a personal computer, and contains components such as a mainbody 2001, a casing 2002, a display device 2003, and a keyboard 2004.The present invention can be used in the display device 2003.

FIG. 17B is a video camera, and contains components such as a main body2101, a display device 2102, a sound input portion 2103, operationswitches 2104, a battery 2105, and an image receiving portion 2106. Thepresent invention can be used in the display device 2102.

FIG. 17C is a portion (right side) of an EL display which is attached toone's head, and contains components such as a main body 2201, a signalcable 2202, a head fixing band 2203, a display monitor 2204, an opticalsystem 2205, and a display device 2206. The present invention can beused in the display device 2206.

FIG. 17D is an image playback device furnished with a recording medium(specifically, a DVD playback device), and contains components such as amain body 2301, a recording medium (such as a CD, an LD, or a DVD) 2302,operation switches 2303, a display device (a) 2304, and a display device(b) 2305. The display device (a) mainly displays image information. Thedisplay device (b) mainly displays character information, and thepresent invention can be used in the display device (a) and in thedisplay device (b). Note that the present invention can be used in imageplayback devices, furnished with a recording medium, such as a CDplayback device and a game machine.

FIG. 17E is a mobile computer, and contains components such as a mainbody 2401, a camera portion 2402, an image receiving portion 2403,operation switches 2404, and a display device 2405. The presentinvention can be used in the display device 2405.

FIG. 17F is an EL display, and contains components such as a casing2501, a support table 2502, and a display device 2503. The presentinvention can be used in the display device 2503. The EL display isadvantageous for cases of making large sized screens because it has awider angle of view compared with a liquid crystal display, and isadvantageous in displays having a diagonal equal to or greater than 10inches (especially for those having a diagonal equal to or greater than30 inches).

Further, if the brightness of the light emitted from EL materialsincreases in the future, it will become possible to use the presentinvention in a front type or a rear type projector by projecting lightcontaining the output image information which is expanded by a lens.

The applicable range of the present invention is thus extremely wide,and it is possible to apply the present invention to electronic devicesof all fields. Furthermore, the constitutions of embodiments 1 to 8 canbe freely combined and used in obtaining the electronic devices ofembodiment 9.

Embodiment 10

An example of manufacturing an active matrix type EL display device byprocesses differing from those of embodiment 1 is shown in embodiment10. FIGS. 18A to 18E are used in the explanation.

First, a base film 1801 is formed with a thickness of 300 nm on a glasssubstrate 1800 in accordance with the processes of embodiment 1. Inembodiment 10, a lamination of silicon nitride oxide films formed insuccession without breaking the vacuum is used as the base film 1801.The concentration of nitrogen contacting the glass substrate 1800 may beset from 10 to 25 wt % at this point.

In addition, an amorphous silicon film (not shown in the figures) isformed with a thickness of 50 nm on the base film 1801 by a known filmdeposition method. The amorphous silicon film is formed in successionafter formation of the base film 1801, without breaking the vacuum. Notethat it is not necessary to limit this film to the amorphous siliconfilm, and that provided that it is a semiconductor film containing anamorphous structure (including microcrystalline semiconductor films),other films may also be used. In addition, compound semiconductor filmscontaining an amorphous structure such as an amorphous silicon germaniumfilm may also be used. Further, the film thickness may be set from 20 to100 nm.

The amorphous silicon film not shown in the figures is crystallized nextby employing excimer laser light using XeCl gas. The laser lightcrystallization process is also performed in succession after formationof the amorphous silicon film without breaking the vacuum. A crystallinesilicon film 1802 is thus formed.

In addition, a first gate insulating film 1803 is formed on thecrystalline silicon film 1802 with a thickness of 5 to 100 nm(preferably between 10 and 30 nm). A silicon oxide film is used as thefirst gate insulating film 1803 in embodiment 10. The first gateinsulating film 1803 is also formed in succession after forming thecrystalline silicon film 1802 without breaking the vacuum. The state ofFIG. 18A is thus obtained.

The base film formation process, the amorphous silicon film formationprocess, the amorphous silicon film crystallization process (thecrystalline silicon film formation process) and the first gateinsulating film formation process are thus characterized in that all areperformed successively without breaking the vacuum (without exposure tothe atmosphere). This type of successive process can be realized byusing a multi-chamber method (also referred to as a cluster tool method)provided with a plurality of film deposition chambers and a lasercrystallization chamber.

Next, the crystalline silicon film 1802 is patterned byphotolithography, and island shape semiconductor films 1804 to 1807 areformed. (See FIG. 18B.)

A second gate insulating film 1808 is formed next so as to cover theisland shape semiconductor films 1804 to 1807. In a region whichfunctions essentially as a gate insulating film, the first gateinsulating film 1803 and the second gate insulating film 1808 have alamination structure. However, it is preferable to form the first gateinsulating film 1803 with a thin film thickness of 10 to 30 nm, andtherefore the film thickness of the second gate insulating film 1808 maybe regulated within the range of 10 to 120 nm.

Resist masks 1809 a and 1809 b are formed next, and a processing ofadding an n-type conductivity element is performed. This process may beperformed under the same conditions as those of the process of FIG. 4Bin embodiment 1. N-type impurity regions 1810 and 1811 containing ann-type impurity element with a concentration from 2×10¹⁶ to 5×10¹⁹atoms/cm³ (typically 5×10¹⁷ atoms/cm³ to 5×10¹⁸ atoms/cm³) are thusformed. (See FIG. 18D.)

The resist masks 1809 a and 1809 b are next removed, and a process ofactivating the n-type impurity elements is performed. The process ofFIG. 4C of embodiment 1 may be referred to for this process. (See FIG.18E.)

Subsequent processing may be performed in accordance with the steps ofembodiment 1 from FIG. 4E onward. An active matrix type EL displaydevice like that explained by embodiment 1 can thus be manufactured.

Note that the constitution of embodiment 10 can be freely combined withthe composition of any of embodiments 2 to 4, 6, and 7, and that theapparatus of embodiment 8 may be used in manufacturing an EL element.Furthermore, the electronic devices shown in embodiment 9 may use the ELdisplay device manufactured by implementing embodiment 10.

Embodiment 11

An example of manufacturing an active matrix type EL display device byprocesses differing from those of embodiment 1 is shown in embodiment11. FIGS. 19A to 19D are used in the explanation.

In embodiment 11, the technique recorded in Japanese Patent ApplicationLaid-open No. Hei 7-130652 is used in forming the crystalline siliconfilm 302 shown in FIG. 4A of embodiment 1. Namely, nickel is used as acatalytic element which promotes crystallization of an amorphous siliconfilm in embodiment 11. Processes of FIG. 4B onward are then performed,and the state of FIG. 5A is obtained.

Resist masks 1901 a and 1901 b are formed next, and a process of addingan n-type impurity element (phosphorus in embodiment 11) is performed inthis state. FIG. 5B of embodiment 1 may be referred to for the additionconditions at this point. N-type impurity regions 1902 to 1909 are thusformed. (See FIG. 19A.)

The resist masks 1901 a and 1901 b are removed next, and a protectingfilm 1910 is formed. A process of activating the n-type impurityelements added to the n-type impurity regions 1902 to 1909 by furnaceannealing using an electric furnace is then performed. Activation isperformed at 500 to 600° C., and the nickel used in crystallizing thecrystalline silicon film 302 moves to the n-type impurity regions 1902to 1909 by a phosphorus gettering action at this point. The nickelgettering process and the phosphorus activation process are thereforecombined in the process of FIG. 19B.

A resist mask 1911 is formed next, and a process of adding a p-typeimpurity element (boron in embodiment 11) is performed. FIG. 5C ofembodiment 1 may be referred to for the addition conditions at thistime. P-type impurity regions 1912 and 1913 are thus formed. (See FIG.19C.)

An interlayer insulating film 1914 made from a silicon nitride oxidefilm is formed next, and a hydrogenation process is performed in thisstate. Hydrogen within the interlayer insulating film 1914 is made todiffuse within an active layer by heat treatment at 300 to 450° C. inthis hydrogenation process. Further, boron added to the p-type impurityregions 1912 and 1913 is activated at the same time. The hydrogenationprocess and the boron activation process are therefore combined in theprocess of FIG. 19D. The p-type impurity regions are activated at thesame time as hydrogenated, and therefore a phenomenon of the value ofthe off current of a p-channel TFT becoming higher in a region of highgate voltage can be controlled.

Note that the hydrogenation process and the boron activation process mayalso be performed separately. In other words, after the step of FIG.19C, the boron activation process may be performed at 500 to 600° C.,and the hydrogenation process can be performed next at 300 to 400° C. Itis preferable to perform this when there are cases in which boronactivation is insufficient because the hydrogenation process temperatureis low.

After thus obtaining the state of FIG. 19D, subsequent processes may beperformed in accordance with the processes of FIG. 6A onward inembodiment 1. Note that the interlayer insulating film 1914 may be aportion of the first interlayer insulating film 336 shown in FIG. 6A. Anactive matrix type EL display device like that explained by embodiment 1can thus be manufactured.

Note that the constitution of embodiment 11 can be freely combined withthe composition of any of embodiments 2 to 7 and 10, and that theapparatus of embodiment 8 may be used in manufacturing an EL element.Furthermore, the electronic devices shown in embodiment 9 may use the ELdisplay device manufactured by implementing embodiment 11.

Reflection of light emitted from an EL layer by a cathode surfacebecomes a diffuse reflection by implementing the present invention, anda problem of an observer's face or the surrounding environment beingreflected in an image display portion of an EL display device can besolved.

Furthermore, it becomes unnecessary to use a high price film such as acircular polarization film, and therefore it is possible to reduce thecost of the EL display device and electronic devices using the ELdisplay device.

1. A display device comprising: a pixel portion comprising a pixelcomprising: a first transistor; a second transistor electricallyconnected to the first transistor; and a light emitting elementelectrically connected to the second transistor; a first gate sidedriving circuit at a first side of the pixel portion; a second gate sidedriving circuit at a second side of the pixel portion; and wherein agate electrode of the first transistor comprises a material which isdifferent from a material of a gate wiring line, wherein an electrode ofthe light emitting element is electrically connected to one of a sourcewiring and a drain wiring of the second transistor through a firstcontact hole, wherein the one of the source wiring and the drain wiringof the second transistor is electrically connected to a semiconductorfilm of the second transistor through a second contact hole, wherein thefirst contact hole and the second contact hole do not overlap with eachother, wherein a silicon nitride layer is positioned over the lightemitting element, wherein a current flows through the first transistorin a first direction, wherein a current flows through the secondtransistor in a second direction, wherein the first direction intersectswith the second direction, and wherein the display device does notcomprise a circular polarizing film.
 2. The display device according toclaim 1, wherein a surface of the electrode of the light emittingelement has projecting portions.
 3. The display device according toclaim 1, wherein a surface of a light emitting layer of the lightemitting element has projecting portions.
 4. The display deviceaccording to claim 1, wherein a surface of another electrode of thelight emitting element has projecting portions.
 5. Anelectroluminescence module comprising the display device according toclaim 1 and a flexible print circuit.
 6. An electronic device comprisingthe display device according to claim 1, comprising an operation switch,a battery, or a recording medium.
 7. An electronic device comprising theelectroluminescence module according to claim 5, comprising an operationswitch, a battery, or a recording medium.
 8. A display devicecomprising: a pixel portion comprising a pixel comprising: a firsttransistor comprising a multi-gate structure; a second transistorelectrically connected to the first transistor; and a light emittingelement electrically connected to the second transistor; a first gateside driving circuit at a first side of the pixel portion; a second gateside driving circuit at a second side of the pixel portion; and whereingate electrodes of the first transistor comprises a material which isdifferent from a material of a gate wiring line, wherein an electrode ofthe light emitting element is electrically connected to one of a sourcewiring and a drain wiring of the second transistor through a firstcontact hole, wherein the one of the source wiring and the drain wiringof the second transistor is electrically connected to a semiconductorfilm of the second transistor through a second contact hole, wherein thefirst contact hole and the second contact hole do not overlap with eachother, wherein a silicon nitride layer is positioned over the lightemitting element, wherein a current flows through the first transistorin a first direction, wherein a current flows through the secondtransistor in a second direction, wherein the first direction intersectswith the second direction, and wherein the display device does notcomprise a circular polarizing film.
 9. The display device according toclaim 8, wherein a surface of the electrode of the light emittingelement has projecting portions.
 10. The display device according toclaim 8, wherein a surface of a light emitting layer of the lightemitting element has projecting portions.
 11. The display deviceaccording to claim 8, wherein a surface of another electrode of thelight emitting element has projecting portions.
 12. Anelectroluminescence module comprising the display device according toclaim 8 and a flexible print circuit.
 13. An electronic devicecomprising the display device according to claim 8, comprising anoperation switch, a battery, or a recording medium.
 14. An electronicdevice comprising the electroluminescence module according to claim 12,comprising an operation switch, a battery, or a recording medium.
 15. Adisplay device comprising: a pixel portion comprising a pixelcomprising: a first transistor; a second transistor electricallyconnected to the first transistor; and a light emitting elementelectrically connected to the second transistor; a first gate sidedriving circuit at a first side of the pixel portion; a second gate sidedriving circuit at a second side of the pixel portion; and wherein agate electrode of the first transistor comprises a material which isdifferent from a material of a gate wiring line, wherein an electrode ofthe light emitting element is electrically connected to one of a sourcewiring and a drain wiring of the second transistor through a firstcontact hole, wherein the one of the source wiring and the drain wiringof the second transistor is electrically connected to a semiconductorfilm of the second transistor through a second contact hole, wherein thefirst contact hole and the second contact hole do not overlap with eachother, wherein a silicon nitride layer is positioned over the lightemitting element, wherein a current flows through the first transistorin a first direction, wherein a current flows through the secondtransistor in a second direction, wherein the first direction intersectswith the second direction, and wherein the display device does notcomprise an optical film.
 16. The display device according to claim 15,wherein a surface of the electrode of the light emitting element hasprojecting portions.
 17. The display device according to claim 15,wherein a surface of a light emitting layer of the light emittingelement has projecting portions.
 18. The display device according toclaim 15, wherein a surface of another electrode of the light emittingelement has projecting portions.
 19. An electroluminescence modulecomprising the display device according to claim 15 and a flexible printcircuit.
 20. An electronic device comprising the display deviceaccording to claim 15, comprising an operation switch, a battery, or arecording medium.
 21. An electronic device comprising theelectroluminescence module according to claim 19, comprising anoperation switch, a battery, or a recording medium.
 22. A display devicecomprising: a pixel portion comprising a pixel comprising: a firsttransistor comprising a multi-gate structure; a second transistorelectrically connected to the first transistor; and a light emittingelement electrically connected to the second transistor; a first gateside driving circuit at a first side of the pixel portion; a second gateside driving circuit at a second side of the pixel portion; and whereingate electrodes of the first transistor comprises a material which isdifferent from a material of a gate wiring line, wherein an electrode ofthe light emitting element is electrically connected to one of a sourcewiring and a drain wiring of the second transistor through a firstcontact hole, wherein the one of the source wiring and the drain wiringof the second transistor is electrically connected to a semiconductorfilm of the second transistor through a second contact hole, wherein thefirst contact hole and the second contact hole do not overlap with eachother, wherein a silicon nitride layer is positioned over the lightemitting element, wherein a current flows through the first transistorin a first direction, wherein a current flows through the secondtransistor in a second direction, wherein the first direction intersectswith the second direction, and wherein the display device does notcomprise an optical film.
 23. The display device according to claim 22,wherein a surface of the electrode of the light emitting element hasprojecting portions.
 24. The display device according to claim 22,wherein a surface of a light emitting layer of the light emittingelement has projecting portions.
 25. The display device according toclaim 22, wherein a surface of another electrode of the light emittingelement has projecting portions.
 26. An electroluminescence modulecomprising the display device according to claim 22 and a flexible printcircuit.
 27. An electronic device comprising the display deviceaccording to claim 22, comprising an operation switch, a battery, or arecording medium.
 28. An electronic device comprising theelectroluminescence module according to claim 26, comprising anoperation switch, a battery, or a recording medium.